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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:03 EST.

1	ChangeLog:
2		Started by Ingo Molnar <mingo@redhat.com>
3		Update by Max Krasnyansky <maxk@qualcomm.com>
4	
5	SMP IRQ affinity
6	
7	/proc/irq/IRQ#/smp_affinity and /proc/irq/IRQ#/smp_affinity_list specify
8	which target CPUs are permitted for a given IRQ source.  It's a bitmask
9	(smp_affinity) or cpu list (smp_affinity_list) of allowed CPUs.  It's not
10	allowed to turn off all CPUs, and if an IRQ controller does not support
11	IRQ affinity then the value will not change from the default of all cpus.
12	
13	/proc/irq/default_smp_affinity specifies default affinity mask that applies
14	to all non-active IRQs. Once IRQ is allocated/activated its affinity bitmask
15	will be set to the default mask. It can then be changed as described above.
16	Default mask is 0xffffffff.
17	
18	Here is an example of restricting IRQ44 (eth1) to CPU0-3 then restricting
19	it to CPU4-7 (this is an 8-CPU SMP box):
20	
21	[root@moon 44]# cd /proc/irq/44
22	[root@moon 44]# cat smp_affinity
23	ffffffff
24	
25	[root@moon 44]# echo 0f > smp_affinity
26	[root@moon 44]# cat smp_affinity
27	0000000f
28	[root@moon 44]# ping -f h
29	PING hell (195.4.7.3): 56 data bytes
30	...
31	--- hell ping statistics ---
32	6029 packets transmitted, 6027 packets received, 0% packet loss
33	round-trip min/avg/max = 0.1/0.1/0.4 ms
34	[root@moon 44]# cat /proc/interrupts | grep 'CPU\|44:'
35	           CPU0       CPU1       CPU2       CPU3      CPU4       CPU5        CPU6       CPU7
36	 44:       1068       1785       1785       1783         0          0           0         0    IO-APIC-level  eth1
37	
38	As can be seen from the line above IRQ44 was delivered only to the first four
39	processors (0-3).
40	Now lets restrict that IRQ to CPU(4-7).
41	
42	[root@moon 44]# echo f0 > smp_affinity
43	[root@moon 44]# cat smp_affinity
44	000000f0
45	[root@moon 44]# ping -f h
46	PING hell (195.4.7.3): 56 data bytes
47	..
48	--- hell ping statistics ---
49	2779 packets transmitted, 2777 packets received, 0% packet loss
50	round-trip min/avg/max = 0.1/0.5/585.4 ms
51	[root@moon 44]# cat /proc/interrupts |  'CPU\|44:'
52	           CPU0       CPU1       CPU2       CPU3      CPU4       CPU5        CPU6       CPU7
53	 44:       1068       1785       1785       1783      1784       1069        1070       1069   IO-APIC-level  eth1
54	
55	This time around IRQ44 was delivered only to the last four processors.
56	i.e counters for the CPU0-3 did not change.
57	
58	Here is an example of limiting that same irq (44) to cpus 1024 to 1031:
59	
60	[root@moon 44]# echo 1024-1031 > smp_affinity_list
61	[root@moon 44]# cat smp_affinity_list
62	1024-1031
63	
64	Note that to do this with a bitmask would require 32 bitmasks of zero
65	to follow the pertinent one.
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